PART V — SEVERE STORMS 



Local convective phenomena are 

 significantly affected by a greater 

 variety of processes and factors than 

 widespread weather, and are corre- 

 spondingly more difficult to model 

 realistically. To date, we have some 

 two-dimensional models that incor- 

 porate simplified formulations of pre- 

 cipitation-related processes and of 

 entrainment. These show some skill 

 in predicting, for example, the maxi- 

 mum height to which a cloud tower 

 rises with specified ambient condi- 

 tions. The most comprehensive of 

 today's models, however, is probably 

 less detailed by a factor of at least 

 100 than one that would illustrate 

 significant features of the asymmetric 

 horizontal and vertical structure. 



Today's mathematical models of 

 the tornado itself treat cylindrically 

 symmetric cases. At the edge of 

 knowledge, we find steady-state mod- 

 els such as Kuo's, which appears to 

 describe essential features of observed 

 tornadoes in terms of an unstable 

 vertical stratification and an ambient 

 field of rotation. The fact that these 

 features are often present when tor- 

 nadoes are absent, however, serves to 

 emphasize that we still have very far 

 to go in our modeling and observing 

 to identify the factors responsible for 

 concentrating angular momentum in 

 the developing tornado. 



Experiments — The control of pa- 

 rameters afforded by laboratory con- 

 ditions recommends the experimental 

 approach to identification and analy- 

 sis of factors responsible for the 

 growth of tornadoes. Such experi- 

 ments have been conducted for many 

 years, often in conjunction with theo- 

 retical investigations, and realistic- 

 appearing vortices have been pro- 

 duced in various liquids and in air 

 under a considerable variety of ex- 

 perimental conditions. The very ease 

 with which tornado-like vortices can 

 be produced experimentally has made 

 it difficult to progress much beyond 

 theoretical implications regarding the 

 development of swirling motion in 

 converging fluid at the base of a ris- 



ing column, and the important influ- 

 ence of boundaries. 



Concurrent with the recent devel- 

 opment of numerical analysis of 

 large-scale atmospheric circulations, 

 however, has come appreciation of 

 the importance of similarity both in 

 theoretical and experimental model- 

 ing. Similarity in flows on different 

 scales is said to exist when the ratios 

 of various quantities involving inertia, 

 viscosity, rotation, and diffusion are 

 the same. Considerations of similar- 

 ity, and increased attention to such 

 natural observations as are available, 

 are leading to design of models more 

 revealing of the effects of natural 

 conditions. 



Thus, Turner and Lilly have con- 

 structed physical models of vortices 

 driven from above to simulate the 

 convection in a cloud, and have found 

 rising motion in the vortex core with 

 descending motion in a surrounding 

 annulus. Ward, noting that no tor- 

 nado vortex can be indefinitely long, 

 has ingeniously separated a fan from 

 the vortex it creates in controlled in- 

 flow beneath. In this model, his con- 

 trol of the inflow angle and depth of 

 the inflow layer represent the most 

 important influences in the creation 

 of a vortex, its intensity and diameter, 

 and, in contrast to earlier models, the 

 development of a central downdraft. 



The problems of developing theo- 

 retical and experimental models in- 

 dicate the importance of observations 

 on even gross characteristics of tor- 

 nado circulations. Is the flow upward 

 or downward in the funnel core? 

 How is tornado behavior, such as 

 funnel-skipping, related to the rough- 

 ness of underlying terrain? What is 

 the wind inflow angle and air pres- 

 sure at various distances from the 

 visual funnel? How does the wind 

 vary with height in the vicinity of 

 tornadoes? If we could better answer 

 these questions for atmospheric cases, 

 we could design experiments accord- 

 ingly, and rationally extend our 

 search for influential parameters of 

 the flow. 



Comments on Investigational 

 Techniques 



We have surveyed observational, 

 theoretical, and experimental aspects 

 of tornado investigations. The vari- 

 ety and complexity of processes im- 

 plicit in tornado development and 

 maintenance, and the rarity, relatively 

 small scale, and intensity of the natu- 

 ral phenomena have been sources of 

 great difficulty. Let us briefly con- 

 sider the helpful technological ad- 

 vances that may reasonably be antici- 

 pated and whose development should 

 be encouraged. 



Emerging Observational Tech- 

 niques — With regard to observa- 

 tions, no available prototype tech- 

 nique seems practical for measuring 

 details of the distribution of velocity 

 and other parameters in a tornado 

 vortex. With the encouragement of 

 severe-storm study programs, how- 

 ever, greater numbers of observations 

 — including useful motion pictures — 

 should become available, and we may 

 reasonably expect an opportunity in 

 the next few years to extend the im- 

 portant study of the Dallas tornado 

 of April 2, 1957, made by Hoecker 

 and his colleagues. 



Emphasis should be placed on ob- 

 serving the circulations around severe 

 storms, since it is certain that the 

 intensity of a storm and the occur- 

 rence of tornadoes is greatly con- 

 trolled by the storm environment. In 

 addition to encouraging existing pro- 

 grams having this objective, we may 

 put special emphasis on two emerg- 

 ing tools. One is meteorological 

 doppler radar, which in units of two 

 or three can map the distribution of 

 precipitation velocity with unprece- 

 dented detail. The development of an 

 improved doppler capability would 

 have value both for fundamental re- 

 search and for research on an im- 

 proved warning system, the latter by 

 providing bases for evaluating the 

 distinguishing features in a storm 

 velocity field characteristic of an im- 

 pending tornado. Doppler capabil- 

 ity for clearer tornado identification 



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